TST: isspmatrix_csr is deprecated in scipy-dev #20164

Example log: https://github.com/astropy/astropy/actions/runs/30549199357/job/90893342732 (ignore the _unwrap stuff being handled separately in #20132 )

Affects nddata tests.

All sparse matrix classes (*_matrix) are being deprecated in favor of sparse arrays (*_array), which have a NumPy-compatible API, e.g. * is elementwise multiplication. See the spmatrix to sparray migration guide https://docs.scipy.org/doc/scipy/reference/sparse.migration_to_sparray.html

The isspmatrix_csr function will be removed no earlier than v2.2.